Recent evidence has strongly suggested that the adult body weight of humans is greatly influenced by heredity (Bouchard et. al., 1985; 1986; Bray; 1981; Stunkard et. al., 1986a; 1986b). Since body weight is principally produced and maintained by eating, the genes that influence body weight probably act by affecting food intake, specifically, the frequency, composition, or size of meals. There have been, however, no systematic studies of hereditary influences on the meal patterns of humans. The proposed research will investigate that aspect of eating behavior is genetically determined by studying and comparing the meal patterns of monozygotic (MZ) and dizygotic (DZ) twins. MZ and DZ twins will be asked to enter into a diary everything they either eat or drink and the time of the meals throughout a consecutive seven day period. The records will then be entered into a computer in coded form for analysis of the nutrient composition of the recorded meals. The similarities and differences between overall nutrient intakes and the microstructure of eating including the meal compositions, the intervals between meals, the meal pattern correlations, and the circadian distribution of eating will be determined separately for the MZ and DZ twins. The heritabilities of these factors will then be estimated based upon the difference between the intra-twin correlations for the MZ and DZ pairs. The proposed study should provide insights into how heredity influences the meal patterns of humans and in turn encodes and maintains body weight. This may suggest the nature of the physiological processes that are genetically influenced and suggest methods of intervention to prevent or treat abnormal body weight or eating behavior.